GB2325819A

GB2325819A - Diversity combining RF signals in a mobile radio base station

Info

Publication number: GB2325819A
Application number: GB9806950A
Authority: GB
Inventors: Sheldon K Meredith
Original assignee: Radio Frequency Systems Inc
Current assignee: Radio Frequency Systems Inc
Priority date: 1997-03-31
Filing date: 1998-03-31
Publication date: 1998-12-02
Also published as: GB9806950D0; SE9800633D0; CA2217813A1; SE9800633L; JPH10303809A

Abstract

A cellular base site having one or more sectors has in each sector a plurality of antennas 102 or antenna beams (from a phase array) for voice channel communications, each antenna or beam providing coverage in a selected azimuthal direction. The antennas are also used for receiving uplink control channel signals thus ensuring the same coverage for voice ands control channels. The antennas are divided preferably equally into a first group and a second group. All of the RE signals received by the first group are combined to form a first composite signal. All of the RF signals received by the second group are combined to form a second composite signal. The first and second composite signals are provided to separate diversity inputs of a control channel radio 145, and also to the separate diversity inputs of a locate radio 147 used to measure signal strengths for handoff decisions. Each antenna or beam in the first group may be situated between antennas or beams of the second group. A separate antenna 170 is used for downlink control channel signalling.

Description

APPARATUS FOR PROVIDING COMBINED RF SIGNALS IN A MOBILE RADIO BASE SITE The present invention relates to radio telephone systems, and more particularly, to improved use of antenna resources for locate radio and control channel radio operation.

Cellular telephone base sites are typically provided with control channel radios for setting up a call between the base site and a mobile radio. The control channel radios include transceivers for transmitting and receiving control channel signals. The transmit side of the control channel is known as the downlink or forward control channel. The receive side of the control channel is known as the uplink or reverse control channel. A base site can be configured with one or more control channels. Typically, omnidirectional base sites will have only one control channel while sectorized base sites may have one or multiple control channels, e.g., three control channels on three different frequencies. For sites with only one control channel, the forward control channel is typically transmitted on a single omni-directional antenna. Alternatively, it may be simulcast on multiple panel antennas. For sectorized sites using multiple control channels, each sector's forward control channel is typically transmitted by a standard 120 panel antenna into its corresponding sector. This scheme allows control channel patterns to be matched well with voice channels. This scheme does not produce coverage holes for rural or high elevation sites as may happen with simulcasting of a single control channel on multiple panel antennas.

Normally, for the uplink or reverse control channel, either two omnidirectional or two panel antennas per sector are provided for diversity reception. When simulcasting with 1200 panels on the downlink, received signals from these panels are combined on the uplink to provide a pseudo-omni receive pattern.

A prior art base site having a single control channel is illustrated in Fig. 1. For setting up a call, a mobile unit 10 measures the various downlink control channels 12 being transmitted by various base sites 20. Each base site 20 includes at least one control channel radio 15 for transmitting a downlink control channel 12 via an omnidirectional antenna 19. The mobile unit 10 selects a particular base site 20 based on various parameters such as control channel frequency, signal strength and base site preferences. The mobile unit 10 then transmit on the reverse control channel 22 for the selected base site 20. The control channel radio 15 is typically a diversity receiver, and two omni-directional antennas 25, 26 are provided to receive the reverse control channel 22. The control channel radio 15 measures the uplink diversity of the reverse control channel 22. This signal measurement is also provided to the mobile telephone switching office (MTSO) 30 via control system equipment 32 at the base site. The base site control system 32 and MTSO 30 will validate the user and determine a "go to channel" indicative of the channel that the MTSO and/or base site has selected the mobile unit to communicate on. A "go to channel" message is then sent to the mobile unit 10 on the downlink control channel 12.

The mobile unit 10 retunes to the target channel and transmits without a supervisory audio tone (SAT) while the radio channel unit 35 corresponding to the target channel at the target base site transmits a carrier frequency modulated with the SAT on the target channel. The base site 20 measures the signal strength of the signals being transmitted by the mobile unit 10 while the mobile unit 10 measures the SAT from the base site radio channel unit, and transponds the same SAT and opens the audio path. The base site radio then measures the SAT from the mobile unit and opens the audio path.

Once communications are established between a mobile unit 10 and the base site 20, different radios are used for call processing and signal strength measurements, and the control channel radios 15 are no longer used for call processing. The dedicated radios that measure radio frequency (RF) signal strengths for call processing decisions are sometimes called locate radios 37, scans or mobile locators. For purposes of the present invention, these radios will be referred to as locate radios. Those locate radios 37 are also diversity receivers and use the same omnidirectional antennas 25, 26 as the control channel radios 15 for monitoring signal strength.

In some base site configurations, the locate radios 37 are used during call processing to continuously monitor call activity between the mobile unit 10 and the base site 20. In other base sites, the locate radios 37 are only used during a handoff of the mobile radio unit to another base site.

During a call, if the mobile unit 10 begins to move out of range of the base site 20 that it is communicating with, as measured by either the locate radio 37 or the radio channel unit 35 itself, the base site 20 will initiate a handoff query to other base sites via the MTSO 30. The other base sites will also measure the mobile unit's power. When another base site measures the mobile unit's power with a high enough power level difference, then a "go to channel" message is sent to the mobile unit 10 on the downlink voice channel to instruct the mobile unit to go to the appropriate frequency and channel associated with the other base site.

The mobile unit 10 then retunes to the new base site channel, thereby effecting the handoff process. At this point. the mobile unit 10 has ended its communication with the initial base site.

It is known in the art to use a plurality of antennas or antenna beams for transmission and reception of RF signals at a cellular base site. For example, as described in our copending U.S. Patent Application Serial No.

08/708,130 filed on July 26, 1996 entitled "Radio Signal Scanning and Targeting System for Use in Land Mobile Radio Base Sites, " a multi-beam phase array antenna may be used at a base site for communication between the radio channel units 35 of the base site and mobile units 10. In such a configuration, arrays of co-linear radiating elements from each phase array antenna are driven by a phase array feed network so as to generate a plurality of beams for the reception or transmission of radio frequency energy in a particular azimuthal direction. In such a system, a radio channel unit is interconnected to the antenna (antenna beam) which, on average during a sampling period, has the strongest received signal strength of RF signals at the operating frequency of the radio channel unit. However, in such prior art systems, both the locate radios and control channel radios typically use omni-directiona or panel antennas, as described above to thereby provide a large coverage area for the locate radios and control channel radios. However, a problem associated with omni-directional antennas is that when an omni-directional antenna is mounted within close proximity to another antenna or a metal object, which is typically the case at a cellular base site, the radiation pattern of the omni-directional antenna may be severely affected. For example, a "ripple" in the receiving pattern of an omni-directional antenna may occur in the area of another antenna or structural object, such as structural members of a base site tower.

Because of the problems associated with omnidirectional antennas, the locate radios and control channel radios do not necessarily have the same coverage area as the voice channel radios which are utilizing the individual beams of a multibeam antenna for communication. There therefore exists a need for improved antenna coverage for locate radios and control channel radios such that the coverage area is the same as the coverage area for the multiple voice channel radios utilized in a cellular base site.

The invention seeks improved antenna coverage for locate radios and control channel radios in a land mobile radio base site.

The invention may enable antenna coverage for locate radios and control channel radios that is the same as the coverage area for the multiple voice channel radios utilized in a cellular base site.

The invention may permit the provision of an antenna configuration at a land mobile radio base site which reduces the number of antennas required to perform desired communications, reduces wind loading, improves appearance, and reduces installation and maintenance costs.

According to the invention there is provided an apparatus for providing combined RF signals in each sector of a land mobile radio base site, the base site including at least one sector, comprising a plurality of antenna means in a respective sector each for receiving RF signals, said plurality of antenna means being divided into a first group including selected ones of said plurality of antennas means in the respective sector and a second group including remaining ones of said plurality of antenna means in the respective sector and combiner means including means for combining all of the RF signals received on said first group of antenna means to form a first composite signal and means for combining all of the RF signals received on said second group of antenna means to form a second composite signal.

The invention enables the provision of a cellular base site having a plurality of antennas and/or antenna beams in at least one sector for communication by voice channel radios, all of the antennas and/or antenna beams within each sector are divided into a first group including selected ones of the plurality of antennas and/or antenna beams and a second group including the remaining antennas and/or antenna beams not in the first group, and wherein all of the RF signals received on the first group of antennas and/or antenna beams within a sector are combined to form a first composite signal and all of the RF signals received on the second group of antennas and/or antenna beams within a sector are combined to form a second composite signal, and wherein the first composite signal is provided to one diversity input of a control channel radio and a locate radio, and the second composite signal is provided to the other diversity input of the control channel radio and the locate radio.

Half of the antennas and/or antenna beams within each sector may be included in the first group.

Each antenna or antenna beam may provide coverage in a selected azimuthal direction, and the number and beam width of the antennas and/or antenna beams may be selected and arranged to provide 3600 of coverage for the base site, and such that each antenna and/or antenna beam in the first group within each sector is positioned between two antennas and/or antenna beams in the second group within each sector.

Each antenna or antenna beam may provide coverage in a selected azimuthal direction, and the number and beam width of the antennas and/or antenna beams may be selected and arranged to provide 3600 of coverage for the base site, wherein each antenna and/or antenna beam is consecutively numbered starting from a selected first antenna and/or antenna beam, and wherein even numbered antennas and/or antenna beams within each sector are in one group and odd numbered antennas and/or antenna beams within each sector are in the other group.

The present invention provides a significant improvement over the prior art. By utilizing the present invention, the control channel radios and locate radios utilized in a cellular base sites may have the identical coverage area as the voice channel radios utilized in the cellular base site. Therefore, the problems associated with an omni-directional antenna of having a ripple in the area of another antenna or structural object which is in close proximity to the omnidirectional antenna is eliminated. The invention can avoid any requirement for additional omnidirectional antennas or panel antennas for locate radios and for the reverse control channel associated with control channel radios. Reducing the number of antennas at a particular cellular base site provides the advantages of reduced wind loading, improved appearance, and reduced installation and maintenance costs.

In order that the invention and its various other preferred features may be understood more easily, some embodiments thereof will now be described, by way of example only, with reference to the drawings.

Fig. 1 is a schematic block diagram of a prior art base site showing communications between the base site and a mobile radio, Fig. 2 is a schematic block diagram of a base site in accordance with the present invention, Fig. 3 is a more detailed schematic block diagram of a subspace beam combiner utilized in the base site of Fig. 2 and Fig. 4 is a schematic diagram of a beam pattern of the antenna utilized in the base site of Fig. 2.

The present invention is particularly well-suited for use in a base site 100 of a mobile telephone system, such as a cellular telephone system. In particular, the invention is particularly useful in such a base site having a multi-beam phase array antenna including arrays of collinear radiating elements which are driven by a phase array feed network so as to generate multiple beams for the reception or transmission of radio frequency energy in a particular azimuthal direction, such as the base site described in our copending U.S. Patent Application Serial No. 08/708,130 filed on July 26, 1996, the disclosure of which is incorporated herein by reference.

Referring to Fig. 2, the base site 100 is illustrated as including a plurality of antennas 102. Although Fig. 2 illustrates the antennas as individual antenna units such as twelve different narrow-beamwidth antennas, it will be understood by those skilled in the art that a variety of different antenna configurations may be used with the invention. The antennas can be thought of as separate signal ports for interconnection with transceivers. As an alternative to 12 physically different narrow-beamwidth antennas, each antenna 102 may actually be a particular beam of a multi-beam phase array antenna system, wherein arrays of co-linear radiating elements form each phase array antenna, with the arrays driven b. a phase array feed network, so as to generate a plurality of beams or lobes, each of which acts as a separate signal port for the reception or transmission of radio frequency energy in a particular azunuthal direction. Additionally, although 12 antennas or antenna beams are shown, different numbers of antennas or antenna beams may be used in different system configurations. In another embodiment of the invention, 16 antenna beams of a multi-beam phase array antenna are used.

However, regardless of the number of antennas or antenna beams used, the present invention is equally well understood as described herein with respect to a base site having 12 different antennas or antenna beams.

The antennas 102 are used for the transmission and reception of RF signals, and a duplexer 106 of a type known in the art is provided for each antenna 102, for allowing each antenna to either transmit or receive RF signals at any one time. The base site includes a plurality of radio channel units 110 (voice channel radios), such as sixty radio channel units, for communication via the antennas 102.

The radio channel units 110 may be interfaced with the antennas 102 via a control system 115 which dynamically interconnects each radio channel unit 110 with the antennas 102 determined to have the strongest received signal strength of RF signals at the operating frequency of the radio channel unit 110. Such a system is described in the aforementioned copending U.S. Patent Application Serial No.

08/708,130, the disclosure of which is incorporated herein by reference.

For the transmission of RF signals from a radio channel unit 110, the control system 115 determines the most suitable antenna 102 and thereafter provides the RF signals from the radio channel unit 110 via an amplifier 118 and a bandpass filter 120 to the duplexer 106 such that the RF signals are transmitted by the antenna 102 via a forward voice channel 122 to mobile telephone unit 125. RF signals transmitted by the mobile telephone unit 125 via a reverse voice channel 127 are received by the antenna 102 and provided via the duplexer 106, bandpass filter 132, and amplifier 135 to a splitter 130. The splitter 130 provides a portion of the received RF signal to the control system 115. The control system then provides the received RF signals to the appropriate radio channel unit 110. As described in our copending U.S. Patent Application Serial No. 08/708, 130, the disclosure of which is incorporated herein by reference, if the radio channel unit is a diversity receiver, the control system 115 interconnects the two diversity inputs of the radio channel unit 110 with the two antenna determined to have the strongest signal strength of RF signals at the operating frequency of the radio channel unit. Although the invention is described as using bandpass filter 120, 132, such filtering is not necessarily essential.

The antennas 102 may also be utilized to provide reverse control channel signals 140 to control channel radios 145 and the reverse voice channel signals 127 to locate radio 147. A portion of all of the RF signals received by the antennas 102 are provided by the splitters 130 to a subspace beam combiner 150. Referring also to Fig.

3, the subspace beam combiner 150 includes a pair of RF signal combiners 155, 156. The RF signal combiners may be of any suitable type know in the art, such as Wilkenson combiners. Referring to Fig. 4, each of the antennas 102 has a narrow-beamwidth beam directed in a specific azimuthal direction. The twelve narrow-beamwidth beams of the antennas 102 illustrated in Fig. 2 create the beam pattern illustrated in Fig. 4. For purposes of describing the present invention, the beams are consecutively numbered with a beam number corresponding to the number of the antenna illustrated in Fig. 2.

As previously described in the background of the invention, both the locate radio 147 and the control channel radio 145 are diversity receivers having two inputs for received RF signals such that the locate radio and the control channel radio select between the two diversity inputs based on the signal strength of the received signal at the frequency of interest. In order to provide diversity inputs 162 of the locate radio 147 and diversiy inputs 165 of the control channel radios 145, the subspace beam combiner 150 combines alternating beams using the two RF signal combiners 155, 156. Therefore, as illustrated in Fig.

3, with the antenna beams consecutively numbered between 1 and 12, the odd numbered beams are combined by one of the RF signal combiners 155 to provide a first combined signal at the diversity 1 output and the even numbered beams are combined by the RF signal combiner 156 to provide a second combined signal at the diversity 2 output. Within the RF signal combiners, each of the signals from the various antenna beams are equally combined.

For transmission of the forward control channel, an omnidirectional antenna 170 is interconnected to the control channel radio 145 for transmitting of the forward control channel 175 to the mobile telephone unit 125.

The operation of the invention is best understood by example. The control channel radio 145 of each base site 100 transmits a forward control channel 175 via the omnidirectional antenna 170. A mobile telephone unit 125 desiring to establish communications monitors each of the control channels 175 being transmitted by the various base sites 100. The mobile unit selects a particular base site based on various parameters such as control channel, signal strength and various base site preferences. Once the mobile unit 125 selects a base site to establish communications with, the mobile unit transmits on the reverse control channel 140 to initiate a request. (Alternatively, the mobile radio unit 125 will transmit on the reverse control channel 140 in response to a page when a base site contacts the mobile radio unit 125 for receiving an incoming call.) The reverse control channel 140 is received on the various antenna 102 of the base site.

Depending on the location of the mobile telephone unit 125 with respect to the base site 100, certain of the directional beams (Fig. 4) will receive a stronger signal strength of the reverse control channel 140 than other of the antenna beams. A portion of all of the RF signals received by the antennas 102 are provided via the duplexer 106, bandpass filter 132, amplifier 135 and splitter 130 to the subspace beam combiner 150. As previously described herein, half of the received RF signals are combined by one of the RF signal combiners 155 to provide the diversity 1 output, and the remaining RF signals are combined by the other RF signal combiner 156 to provide the diversity 2 output. These diversity outputs of the subspace beam combiner 150 provide the diversity inputs 165 to the control channel radio 145, the diversity inputs 162 to the locate radio 147, and these diversity outputs are also provided to the control system 115. The control channel radio will detect the reverse control channel transmitted by the mobile radio unit 125 on either the diversity 1 output or diversity 2 output and provide a measurement of the up link diversity of the reverse control channel 140. This measurement is utilized by the control system 115. Additionally, the measurement is provided by the control system 115 to a mobile telephone switching office (MTSO) 180.

The base site 100 and the MTSO 180 will validate the user of the mobile telephone 125 and determine a channel associated with one of the radio channel units 110 that the mobile telephone unit 125 should communicate on. The control channel radio 145 then transmits a "go to channel" message to the mobile telephone unit 125 via the forward control channel 175. The mobile unit then returns to the target voice channel and transmits on the reverse voice channel 127. At the same time, the base site transmits a supervisory audio tone (SAT) on the forward voice channel 122. The reverse voice channel signal 127 transmitted by the mobile telephone unit 125 is received by the antennas 102 and provided via the duplexers 106, bandpass filter 132, amplifier 135 and splitters 130 to the subspace beam combiner 150 for providing the diversity inputs 162 to the locate radio 147. The locate radio 147 measures the signal strength of the signals being transmitted by the mobile unit 125 while the mobile unit 125 measures the SAT transmitted on the forward voice channel 122 from the base site 100.

Thereafter, the mobile unit 125 transmits the same SAT and an audio path is opened between the mobile telephone 125 and the base site 100.

Once communications are established between the mobile telephone unit 125 and the base site 100, the radio channel unit 110 and/or the locate radio 147 continues to measure the signal strength of the reverse voice channel 127 from the mobile telephone unit 125. These signal strength measurements are provided to the control system 115 and the MTSO 180. If the mobile unit 125 begins to move out of range of a base site 100 that it is communicating with, as measured by either the radio channel unit 110 and/or the locate radio 147, the base site 100 will initiate a handoff via the MTSO 180 to another base site which is more suitable for communication with the mobile telephone unit 125.

In some mobile radio base sites, during time when a radio channel unit 110 is not communicating with a mobile radio unit 125, it will monitor for background levels of RF signals at is operating frequency. This monitoring is often referred to a foreign carrier detection or radio sealing. In order to perform such monitoring, RF signals (background) received on the antennas 102 are provided via the duplexers 106, bandpass filter 132, amplifier 135 and splitters 130 to the subspace beam combiner 150. The outputs of the two RF signal combiners 155, 156 are provided to the control system 115 for providing the diversity inputs to the radio channel units that are not communicating with a mobile radio unit for performing the desired signal measurements.

The invention has been described herein with respect to an omni-directional base site having a single control channel wherein any radio channel unit may be interconnected to any antenna (antenna beam). However, the principles of the present invention are equally applicable to a sectorized base site having a separate control channel for each sector with specific radio channel units assigned to each sector.

For example, in a base site having 12 antennas or antenna beams in three sectors of 1200 each, there are four antennas or antenna beams in each sector. In such a configuration, two of the antennas in each sector are combined to provide the first composite signal and the remaining two antennas in the sector provide the second composite signal. As discussed above, adjacent antennas (antenna beams) are not combined, instead, every other beam is combined in each sector to provide the first and second composite signals. In another alternative, the radio channel units may be sectorized, i.e., each radio channel unit (voice channel) assigned to a specific sector, with a single omni-directional control channel radio as previously described herein.

Claims

Claims:

1. An apparatus for providing combined RF signals in each sector of a land mobile radio base site, the base site including at least one sector, comprising a plurality of antenna means in a respective sector each for receiving RF signals, said plurality of antenna means being divided into a first group including selected ones of said plurality of antennas means in the respective sector and a second group including remaining ones of said plurality of antenna means in the respective sector and combiner means including means for combining all of the RF signals received on said first group of antenna means to form a first composite signal and means for combining all of the RF signals received on said second group of antenna means to form a second composite signal.

2. An apparatus as claimed in claim 1, wherein each sector includes at least one control channel radio having a pair of diversity inputs, and wherein the first composite signal is provided to one diversity input of the control channel radio and the second composite signal is provided to another diversity input of the control channel radio.

3. An apparatus as claimed in claim 1 or 2, wherein the land mobile radio base site includes at least one locate radio having a pair of diversity inputs, and wherein said first composite signal is provided to one diversity input of said locate radio wherein said second composite signal is provided to another diversity input of said locate radio.

4. An apparatus as claimed in any one of claims 1 to 3, wherein each sector includes a plurality of voice channel radios each having a pair of diversity inputs, and wherein the first composite signal is provided to one diversity input of each voice channel radio in the respective sector wherein said second composite signal is provided to another diversity input of each voice channel radio in the respective sector.

5. An apparatus as claimed in claim 4, wherein the first and second composite signals are provided to the pair of diversity inputs of a respective voice channel radio only when the respective voice channel radio is not communicating with a mobile radio unit.

6. An apparatus as claimed in any one of the preceding claims, wherein half of the antenna means are included in the first group.

7. An apparatus as claimed in any one of the preceding claims, wherein each antenna means provides coverage in a selected azimuthal direction, and the number and beam width of each antenna means for every sector are selected and arranged to provide 360 of coverage for the base site, and wherein each antenna means in the first group is positioned between two antenna means in the second group.

8. An apparatus as claimed in any one of claims 1 to 6, wherein each antenna means provides coverage in a selected azimuthal direction, and the number and beam width of each antenna means for every sector are selected and arranged to provide 3600 of coverage for the base site, wherein each antenna means is consecutively numbered starting from a selected first antenna means, and wherein even numbered antenna means are in said first group and odd numbered antenna means are in said second group.

9. An apparatus as claimed in any one of the preceding claims, wherein each antenna means is a beam of a multi-beam phase array antenna system, each beam acting as a separate signal port for the reception or transmission of radio frequency energy in a particular azimuthal direction.

10. An apparatus as claimed in any one of claims 1 to 8, wherein each antenna means is an individual antenna for the reception or transmission of radio frequency energy in a particular azimuthal direction.

11. An apparatus for providing combined RF signals in each sector of a land mobile radio base site substantially as described herein with reference to Figures 2 to 4 of the drawings.